The present invention relates to a low voltage-high voltage converter of the flyback type.
In many cases where only a source of low voltage is available, for example in the case where equipment is airborne, a high-voltage supply may nevertheless be required. This is the case, for example, for munitions in which the firing is done by an igniter which, for firing, requires a current pulse of large amplitude delivered in a very short time (for example, a current of 3 to 5 kA in a time between 0.1 and 0.5 .mu.s). In that case, a high-voltage supply system is required, charging an energy reservoir such as a capacitor which must then be discharged very quickly, for example by means of a static commutator such as a controlled discharger.
In order to produce this high voltage and to charge the capacitor, a low voltage-high voltage converter is used, which converts the on-board voltage into high voltage. For reasons of practicability of the transformer of the converter, that is to say in order to be able to use an easily producible transformer ratio, as will be seen in more detail in the rest of the description, it has appeared to be necessary to utilized a low voltage-high voltage converter of the flyback type. This type of converter profits from the overvoltage obtained when the primary circuit of the transformer is opened with the aid of a switch.
The principle of these converters is that energy is accumulated in the magnetic core of the transformer during the whole of the time T.sub.on when the switch in the primary circuit is closed and that this energy is recovered in the secondary circuit in order to charge a capacitor during the time T.sub.off when the switch is open.
In most of the known convertors of this type, the on time T.sub.on of the switch is constant and the off time T.sub.off is modulated with the aid of a secondary regulation loop which operates by comparing the output voltage obtained with a reference value.
However, despite its advantages, a converter of this type presents problems in production when the low-voltage supply voltage can vary within wide limits, as is particularly the case when the source of energy is a cell or a battery which supplies a widely variable voltage according to its more or less discharged state. In fact, in this case, as the time T.sub.on is constant in principle, the primary energy accumulated varies in large proportions and thus also the charge time of the capacitor on each cycle. This thus leads to an unfavourable sizing of the converter which must be adapted to a maximum charge time when the supply voltage is a minimum, the component stressed the most being the transformer for which it is also necessary to avoid reaching saturation when the supply voltage is at its maximum value.
To overcome this drawback, it has been proposed in the patent U.S. Pat. No. 3,916,281 to make such a converter work with a constant primary energy accumulated in the transformer during each ON time T.sub.on. For that reason, the patent mentioned above proposes to regulate this ON time as a function of the primary current flowing in the primary winding of the transformer. This mode of regulation permits the transformer to work substantially within rated conditions whatever the value of the supply voltage.
However, especially in the case of the utilization of the high voltage for the firing of ammunition, it can be very important that the charge time of the energy reservoir capacitor is perfectly constant and known in advance whatever the values of parameters such as the actual supply voltage or the temperature, in particular for reasons of safety.
Moreover, in such an application where it is necessary not to create high current demands on the supply which is common to several equipments of various kinds, the transformer is required to have a low inductance. That gives rise to very short T.sub.on times (for example, of the order of a microsecond or even less). Thus, if a closed-loop servocontrol system is used on the primary current, it is necessary to provide, in order to set up the loop, electronics (operational amplifier, etc.) with high performance levels in terms of speed. This entails complex equipment which is awkward to implement and thus costly.